Fluorescent Short-Stranded Helical Foldamers Based on L-shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine

Helical structures were constructed by using π-spacer-bridged dimers of dibenzopyrrolo[1,2-a][1,8]naphthyridine, which has a highly fluorescent L-shaped π-extended skeleton. Three dimers with biphenylene (dimer 1), phenanthrene (dimer 2), and m-phenylene (dimer 3) spacers, as well as a fixed-helical dimer 4 where two quinolone rings were covalently cross-linked, were designed and prepared. ¹ H NMR and ROESY spectra revealed that dimers 1 and 2 adopted helical forms in solution, whereas dimer 3 did not. The helical conformation of 1 was strengthened by addition of either polar or nonpolar solvents to the chloroform solution, which suggested that π-π stacking was the main contributor to the stabilization of the helical structure. All of the dimers, including fixed-helical dimer 4, emitted fluorescence with high quantum yields (ϕ=0.79-0.86).